1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus or a printer, and more particularly to an image forming apparatus in which the heater of the fixing device does not require electric power supply while in a standby state waiting for the input of a print signal.
2. Related Background Art
In the image forming apparatus such as copying apparatus, printer or fascimile, as the thermal fixation device for thermally fixing an unfixed toner image, formed by a suitable image forming process such as electrophotographic process, electrostatic recording process or magnetic recording process corresponding to desired image information and deposited on a recording material (sheet member such as a transfer sheet, a photosensitive paper, an electrostatic recording paper or a printing paper) by a transfer method (indirect method) or a direct method, there is widely employed a device of contact heating type utilizing a heating roller or a heating film.
In particular, the Japanese Patent Application Laid-open Nos. 63-313182, 2-157878, 4-44075 and 4-204980 propose a thermal fixation method utilizing the film heating method in which a film is provided between a heater and a pressure roller for fixing the toner image on the recording material, thereby dispensing with the electric power supply to the heat fixation device in the standby state and minimizing the electric power consumption.
FIG. 14 is a schematic view showing the configuration of the principal portions of such device, in which provided are a heating member (hereinafter called heater) 11 fixed and supported by a stay holder (support member) 12, and an elastic pressure roller 20 pressure contacted with the heater 11 via a heat-resistant thin film (hereinafter called fixing film) 13, forming a nip portion (fixing nip) N of a predetermined nip width. The heater 11 is heated and maintained at a predetermined temperature by electric power supply. The fixing film 13 is a cylindrical or endless belt-shaped member, or a rolled web-shaped member, transported in a direction shown by arrow a, by unrepresented drive means or by the rotating force of the pressure roller 20, in sliding contact with the surface of the heater 11 at the fixing nip portion N.
In a state in which the heater 11 is adjusted to the predetermined temperature and the fixing film 13 is transported in the direction indicated by the arrow, a recording material P constituting a material to be heated and bearing an unfixed toner image t is introduced in the fixing nip portion N between the fixing film 13 and the pressure roller 20, whereby the recording material P is in close contact with the surface of the fixing film 13 and is transported, together with the fixing film 13, in the fixing nip portion N. In the fixing nip portion N, the recording material P and the toner image t are heated by the heater 11 through the fixing film 13 whereby the toner image t on the recording material P is thermally fixed thereto. The portion of the recording material, that has passed the fixing nip portion N, is peeled off from the surface of the fixing film 13 and is transported.
The fixing device of such film heating method can employ, as the heating member, a so-called ceramic heater having a low heat capacity and showing fast temperature rise, and, as the film 13, a thin film also of a low heat capacity, and can realize quick starting in comparison with other heating devices such as of heating roller type, because heating is only required in the nip portion N formed between the heater 11 and the pressure member 20 across the film 13. Therefore, such fixing device is very practical realizing on-demand thermal fixation.
Also the electric power consumption is very low, because the heater need not be powered in the standby state waiting for the entry of the print signal.
However, the pressure roller becomes cold in the standby state because the heater is not powered. Also the temperature is often low immediately after the start of power supply to the image forming apparatus, particularly in a low-temperature atmosphere.
In case the printing operation is started while the pressure roller is cold, the heater is heated to the predetermined temperature capable of fixation immediately after the start of power supply, but the pressure roller shows slower temperature elevation. It is found that, when a first paper sheet is introduced into the nip, the moisture contained in paper evaporates by the heat of the heater and the evaporated moisture condenses and forms dew on the surface of the still cold pressure roller. It is also found that such condensed dew causes slippage of the second and ensuring sheets, thereby inducing unevenness in the image fixation.